I. Field of the Invention
The invention relates to a multi-channel filter.
The invention further relates to an acoustic echo canceller, a noise canceller, a voice controlled audio and/or video system, a audio and/or video conferencing system and to a multi-channel filtering method.
II. Description of the Related Art
Multichannel hands-free operation is becoming an increasingly important feature in modern communication systems. In audio and/or video conferencing systems, multi-channel transmission leads to a better ‘localization’ of the people in the rooms. This enhances the intelligibility and naturalness of the speech. Another important recent development is the development of voice controlled stereo (or multi-channel) audio- and video-equipment. The recognition rate of a voice recognition engine drops dramatically due to the sound emitted by the equipment. Besides that, it may recognize words emitted by the equipment itself. There is, therefore, a need for a multi-channel filter which functions as an acoustic echo canceling preprocessor for the voice recognition engine.
Such a multi-channel filter may be created by simply combining a number of single channel adaptive filters. However, such a multi-channel filter, in general has a very poor performance.
When the number of input signals in a multi-channel filter is larger than the number of independent signal (and noise) sources, there is no longer a unique solution for the adaptive filters. This is the so-called ‘non-uniqueness’ problem. In practice, however, the number of independent sources is always larger than the number of microphones due to noise and other disturbances. When, however, the power is not equally distributed over the independent sources, the problem might get badly conditioned, which is the main reason for the relatively bad performance of multi-channel adaptive filters. Methods have been proposed to increase their performance by using Recursive Least Squares (RLS)-like algorithms (that have a huge computational complexity, even the most efficient implementations), or by adding noise to the input signals, or by processing them in a non-linear way (which may lead to audible artifacts in the speaker signal(s)).
Similar problems arise in multi-channel noise cancellation, where noise that is picked up by a microphone is reduced using extra microphone signals as noise reference signals. The reference signals are filtered and subtracted from the (delayed) primary microphone signal.
In European Patent Application No. 1 052 838, corresponding to U.S. Pat. No. 6,738,480, a stereo frequency domain adaptive filter is disclosed. This multi-channel filter comprises two adaptive filters (one for each channel) for filtering the two input signals. The filtered signals are combined by means of an adder, and the combined signal is supplied to an output of the multi-channel filter. In the update of the two adaptive filters, transformed inverse auto- and cross-correlation matrices are used to improve the performance. This improves the convergence behavior without having a huge influence on the computational complexity compared to using two single channel adaptive filters.
The known multi-channel filter is relatively complex in that a relatively large number of filter coefficients need to be stored in memory.